Residual stresses caused by uneven cooling of seamless square tubes
. Residual stress is the internal self-equilibrium stress in the absence of external force. Hot-rolled steel of all kinds of cross-section has such residual stress. The larger the general section size of steel, the greater the residual stress. Although the residual stress is self-balanced, the performance of steel components in the external force or have a certain impact. Such as deformation, stability, fatigue and other aspects may have adverse effects.
Draw the seamless square tube process: Because the size of the extubation die hole is smaller than the outer diameter dimension of the seamless square tube before drawing (except expansion and contraction), in order to make the front end of the seamless square tube smoothly in order to make the clamping jaws of the drawn car clamp the head of the seamless square tube, the head of the seamless square tube needs to be pointed before the drawing to this end, the seamless square tube should be heated in the seam furnace a certain length of the tube end, usually 150 ~ 200mm, according to the different wall thickness heating temperature between 750 ~ 900 ℃. Then use 75 ~ 300 kilograms of air hammer hit the tip, grind thin seamless square tube head, usually less than 22mm diameter seamless square tube available forging hammer or roll forging machine tip.
Seamless square tube in order to improve the mechanical properties of ductile iron, the general castings are heated to above Afc1 30 ~ 50 ℃ (Afc1 on behalf of the formation of the end of the heating temperature A), quenched into the oil after heating, to obtain martensite. In order to properly reduce the residual stress after quenching, tempering should generally be carried out after quenching, tempering tempered low tempered martensite plus residual bainite plus spherical graphite. This kind of tissue has good abrasion resistance and is used for parts requiring high abrasion resistance and high strength. Temperature tempering temperature 350-500 ℃ after tempering the organization for the tempering tuolite plus spherical graphite, suitable for demanding wear resistance, with a certain effect of stability and flexibility of the thick pieces. High temperature tempering temperature of 500-60D ℃, after tempering the organization for the tempering soxhlet plus spherical graphite, with toughness and strength combined with good overall performance, so widely used in the production.
Therefore, in order to completely eliminate the stress of seamless square tube, the best way is to let it cool evenly during heating.